Wind turbines are an ancient means of extracting the free, in the sense of being fuelless, energy of the wind. They have had a long and successful history of contributing to such tasks as pumping water and milling grain, tasks which could be done intermittently whenever the wind was available to do the work. With the pressure of competition from the internal combustion engine and its cheap power, the wind turbine went into decline in that part of the world economy which either had oil or could afford to import it. By the time of the "oil crisis" of 1973 wind turbines had been driven even from their historic role in the pumping of the Dutch polders.
With the disruptions of the oil supply in 1973, realizations arose that fossil fuels would sooner or later become scarce and expensive. As they did, the price of these fuels would rise and their quality, especially their cleanliness, would fall. Furthermore, economies dependent upon imported fuels would be increasingly at the mercy of fewer and fewer suppliers. With these realizations many of the industrialized nations tried to find alternative sources of energy. The main result of these programs was to discover how difficult it was to find such sources which could compete economically with oil. Of the competing alternates, wind energy appears to have come closest to sucess. In addition to economic factors, it has the further attributes of being free of chemical pollution and the "greenhouse" effect.
The difficulty faced by existing wind-turbines is that their cost of operation, which is almost entirely the cost of the capital to build them, is too high. The principal reason for this difficulty is that the airloads are carried to the output through inefficient bending structures. In addition to the excessive cost and weight of these primary structures, several significant consequences flow from this fundamental characteristic. Economies of large scale cannot be realized because, after reaching some critical size, the cost of the rotating components of wind turbines increases with size at a rate disproportionate to the value of the energy they produce. While this is true for all turbine systems, it is doubly important in the wind energy area because large scale turbines, by reaching higher into the available wind, would capture much more energy from a given site thereby increasing their role in the energy economy. A further obstacle to large size wind-turbines has been the large torques they produce which must be transmitted through heavy and expensive drive components. To avoid these large torques, previous workers in the field have tried to mount secondary turbines on the tips of the blades of their machines. These secondary turbines would absorb the torque produced by the primary turbine and convert it into mechanical energy at high rotational speeds and correspondingly low torques. These attempts have been unsuccessful because of the structural systems required to support these tip turbines were not cost effective.
Wind turbines would profit from the ability to operate at high rotational and tip speeds. This would have several favorable effects: such turbines would, in principle, have blades with narrower chords and hence weigh and cost less; and the higher rotational speed would produce the same power at lower torque lowering the cost and weight of the driveshaft, couplings, gearbox and other components of the drive system. Conventional structural systems have, more or less, reached the point where further increases in the tip speed would result, for reasons of strength, in such substantial increases in the blade thickness as to cause losses in aerodynamic efficiency outweighing any gains.
Conventional vertical axis wind-turbines have rigid blades and must, perforce, use essentially symmetrical airfoils thus losing the advantages of the more efficient cambered airfoils. The blades of present wind turbines require considerable tooling for economic manufacture. As a result, it is prohibitively expensive to customize blades and turbines to each users site. Turbine and blade designs are compromised to average conditions and non-average installations suffer needless performance losses.
Turning to specific examples of the prior art, the leading example of state-of-the-art vertical axis wind turbine technology is the Darrieus turbine named for its inventor G. J. M. Darrieus, who applied for French patents on his machines in 1925 and 1926 and for U.S. Pat. No. 1,835,018. A contemporary design of a Darrieus machine is described in the Sandia National Laboratories Report "Design and Fabrication of a Low Cost Darrieus Vertical Axis Wind Turbine System, Phase II, Volume 2. Final Technical Report" SAND82-7113/2 and related documents. This form of machine, which simplified wind turbine design and construction considerably by eliminating yaw and pitch controls, became the paradigm of the vertical axis wind turbine but is still too expensive to compete unaided against oil. Blades formed into the shape of a "Troposkein" distinguish the Darrieus machine. This shape substantially eliminates the bending moments due to the inertial forces of the rotating blade and carries those loads in tension.
Important disadvantages of the Darrieus turbine include the fact that, since the turbine has curved blades, the blades are expensive to manufacture and are not amenable to variable geometry. As a consequence the Darrieus machine has no aerodynamic control and hence must be designed for unnecessarily high loads. In addition, the expense of the individual blades requires turbines with few, usually two, blades thus precluding the benefits which flow from the smoother operation of several blades. Furthermore the Darrieus rotor is limited to symmetric airfoil sections and loses performance thereby in two ways: it is unable to profit from the superior aerodynamic efficiency of cambered airfoil sections and it is needlessly inefficient in the important neighborhood of zero angle of attack.
The special "troposkein" shape of the rigid Darrieus blades eliminates bending due to the centrifugal loads but leaves the weight and airloads unaffected. Therefore, the blades of the Darrieus machine carry the primary airloads in bending and these loads go through an alternation in sign every revolution, a severe fatigue burden. Furthermore, as the size of the blades increases, the weight loads become a dominant and limiting feature of these and all other rigid blade machines. Too, the rigid blade structure of the Darrieus machine is not amenable to carrying secondary, tip turbines because of the large weight moments such turbines would generate.
Vertical axis machines of the "Savonius" type, i.e. drag driven machines, have intrinsically low efficiencies which various workers have tried to improve. U.S. Pat. No. 4,359,311 (Benesh) teaches the use of a new, rigid bucket shape to improve the aerodynamic efficiency of his Savonius-like machine. U.S. Pat. No. 4,496,283 (Kodric) provides a mechanical means for varying the geometry of a Savonius machine. As mentioned above and discussed below, the present invention involves the use of sail blades and it is noted that the reference in the Kodric patent to sailboats is gratuitous. U.S. Pat. No. 4,156,580 (Pohl) teaches another version of the Savonius machine with rigid blades. None of these teachings deal with actual sail blades.
Others have tried to provide control for lift-driven vertical axis machines. A leading example in U.S. Pat. No. 4,087,202 (Musgrove) which, for example, provides a straight, rigid bladed configuration with variable geometry for control through a complex linkage which must engender unnecessary aerodynamic losses in addition to added costs, maintenance and complexity. This line of attack has also been pursued in U.S. Pat. No. 4,105,363 (Loth) and in U.S. Pat. No. 4,334,823 (Sharp).
The word "sail" was formerly the generic name for what are now termed the "blades". This nomenclature appears in many patent references which show rigid metal structures but refer to them as "sails". An example of this usage is contained in U.S. Pat. No. 4,245,958 (Ewers).
Some patents teach the use of semi-rigid blades. U.S. Pat. No. 4,355,956 (Ringrose et al) teaches the use of a semi-rigid sail fastened along its leading edge to a rigid spar. U.S. Pat. No. 4,561,826 (Taylor) teaches the use of a blade which is elastically supple along its span. Neither of these machines uses a flexible membrane which characterizes a true sail. A particular form of a semi-rigid sail is the "Princeton Sail" advocated by Sweeney and employed in U.S. Pat. No. 4,433,544 (Wells et al) and disclosed by Ahmadi in Wind Engineering Vol.2, No.2, 1978. This sail has a spanwise, rigid, leading edge member about which a double membrane is wrapped. The trailing edge is supported by fine cable. Ahmadi reports his maximum efficiency, i.e. power coefficient, to be 0.008. This, about 1/50 of that of other machines, is too small to be of any use.
Heretofore, vertical axis wind turbines with sail-like blades have had radial beams carrying spars or circular or polygonal frames from which the blades were supported. These configurations were used long ago in China and Iran and one such turbine is in current use in Sweden to drive a carrousel in a children's park.
A photograph of an ancient Iranian turbine of this type is shown in Ahmadi above. A very good example of a Chinese machine of this type is shown in Plate I of the well known book "Generation of Electricity by Wind Power" by E. W. Golding. Such a machine is taught in U.S. Pat. No. 4,052,134 (Rumsey) which shows sailboats with conventionally deployed sails sailing in an annular trough. U.S. Pat. No. 4,342,539 (Potter) shows square sails strung on booms with a telescoping mast. U.S. Pat. Nos. 4,545,729 (Storm) and 4,619,585 (Storm) teach the use of sails mounted on a rotating frame with roller reefing gear to vary the amount of sail deployed and a deformable mechanism to provide variable camber. These constructions gain only a fraction of the advantages of structural efficiency and variable geometry that sail blades can provide and that only at the expense of disadvantages in cost, weight and complexity of the rigid structures and mechanisms required to support them and the aerodynamic inefficiencies that they inevitably entail.
Most operators of wind-turbine installations would welcome systems which are simple, efficient in their conversion of wind energy, inexpensive to build and operate, adaptable to their individual sites, capable of capturing the maximum amount of energy available at their sites, and available in sizes large enough to take full advantage of their sites and are readily controllable.